Polyethylene terephthalate is used on a large scale for the manufacture of food packages such as bottles. Such bottles are widely utilized for packaging of beverages, such as carbonated soft drinks, beer, or mineral water. Whilst some beverage bottlers prefer clear non-pigmented bottles, others prefer colored bottles. Particularly in the case of bottles intended for holding carbonated drinks, a sandwich construction is used in which nylon or an ethylene/vinyl alcohol resin is incorporated in a multi-layer preform with polyethylene terephthalate in order to improve the gas barrier properties of the bottles. It has also been proposed, for the same purpose, to admix a polyamide with the polyethylene terephthalate since the presence of the polyamide provides gas barrier properties.
The technique commonly used to manufacture bottles from molding compositions comprising polyethylene terephthalate generally involves a two stage process. In the first stage granules of the molding composition are injection molded to make a preform. In the second stage the preform is blow molded to the desired shape.
In such a process, the polyethylene terephthalate is typically post-condensed and has a molecular weight in the region of about 25,000 to 30,000 Daltons. However, it has also been proposed to use a fibre grade polyethylene terephthalate, which is cheaper but is non-post-condensed, with a lower molecular weight in the region of about 20,000 Daltons. It has further been suggested to use copolyesters of polyethylene terephthalate which contain repeat units from at least 85 mole % terephthalic acid and at least 85 mole % of ethylene glycol. Dicarboxylic acids which can be included, along with terephthalic acid, are exemplified by phthalic acid, isophthalic acid, naphthalene-2,6-dicarboxylic acid, cyclohexanedicarboxylic acid, cyclohexanediacetic acid, diphenyl-4,4′-dicarboxylic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, and sebacic acid. Other diols which can be incorporated in the copolyesters, in addition to ethylene glycol, include diethylene glycol, triethylene glycol, 1,4-cyclohexanedimethanol, propane-1,3-diol, butane-1,4-diol, pentane-1,5-diol, hexane-1,6-diol, 3-methylpentane-2,4-diol, 2-methylpentane-1,4-diol, 2,2,4-trimethylpentane-1,3-diol, 2-ethylhexane-1,3-diol, 2,2-diethylpropane-1,3-diol, hexane-1,3-diol, 1,4-di-(hydroxyethoxy)-benzene, 2,2-bis-(4-hydroxycyclohexyl)-propane, 2,4-dihydroxy-1,1,3,3-tetramethyl-cyclobutane, 2,2-bis-(3-hydroxyethoxyphenyl)-propane, and 2,2-bis-(4-hydroxypropoxyphenyl)-propane. In this disclosure, the term “polyethylene terephthalate” includes not only polyethylene terephthalate but also such copolyesters.
If the eventual bottle is to be colored, then it is conventional to admix a pigment or pigments with the polyethylene terephthalate granules charged to the hopper of the injection molding machine used to make the bottle preform. For this purpose, the pigment or mixture of pigments can be added as a solid concentrate or in powder form or as a dispersion in a liquid carrier. Such liquid carriers are generally inert materials such as hydrocarbon oils, esters, alcohols, or a mixture of two or more thereof. Any such liquid carrier must be selected so as to have good compatibility with polyethylene terephthalate and, if the pigment is to be dissolved, also good solvent properties for the pigment or pigments. In addition, if the molding composition is to be used for manufacture of food packages, the carrier must be non-toxic. Moreover, the quantity of carrier used should desirably be kept to a minimum so as not to affect adversely the properties of the polyethylene terephthalate in the preform or bottle.
The softening point of polyethylene terephthalate is high. Thus, a typical temperature needed for processing of polyethylene terephthalate is in the region of 260° C. to 285° C. A recognized problem in the industry is that, under the high temperatures and shear conditions needed for injection molding to make a preform and for blow molding of the preform to make a bottle, polyethylene terephthalate tends to degrade, resulting in the formation of acetaldehyde. The presence of acetaldehyde in the material of the finished bottle is undesirable, particularly when the bottle is to be used for products for human consumption, because the acetaldehyde can migrate from the walls of the package or bottle into its contents, whereupon it adversely affects the flavor and fragrance properties of the comestible product. Although the migration of acetaldehyde from a polyethylene terephthalate bottle into a carbonated drink is undesirable, a trace of acetaldehyde can often be tolerated because the taste and fragrance of the drink are not usually noticeably affected. However, the presence of even minute amounts of acetaldehyde in a non-carbonated drink, such as still mineral water, tends to impart a most undesirable adverse taste and odor to the drink.
Methods for measurement of acetaldehyde in industrially injection-molded polyethylene terephthalate preforms have been described by F. Villain et al., Journal of Polymer Science, Vol. 52, 55-60 (1994).
Attempts have been made by equipment manufacturers to modify the design of the processing machinery so as to enable the intensity of the processing conditions needed to make the bottle preforms and for blow molding thereof to be reduced. In this way, it was hoped that the formation of acetaldehyde in the course of the high temperature processing conditions could be minimized.
The use of vented extrudes to devolatilize polymers has been disclosed, for example, in U.S. Pat. No. 5,597,891, which teaches a process for producing reduced acetaldehyde polyester articles by using a purge gas in a vented extruder to remove acetaldehyde. U.S. Pat. No. 5,102,594 discloses a thermoplastic condensation polymer supplied to a vented extruder in powdered form.
In Swiss Patent No. 655,938, there is disclosed a procedure for preparing high molecular weight polyethylene terephthalate containing less than 5 ppm dissolved and bound acetaldehyde which involves treatment of the polyethylene terephthalate with a pure alcohol or alcohol/water mixture at a temperature of at least 130° C. followed by post-condensation at 240° C. to 245° C. in an inert gas or under vacuum.
Another approach that has been attempted is to use additives which will react with the acetaldehyde as it is formed. However, it is important that any additive used should not adversely affect the properties of the bottle or other final product. In particular, it is important not to add a material which can impart undesirable haze or color. Thus, in a paper by F. Villain et al., Polymer Degradation and Stability, 49, 1995, 393-397, it is proposed to incorporate additives in polyethylene terephthalate in order to minimize the amounts of acetaldehyde and formaldehyde produced during the injection molding process. Various stabilizing additives were tested by these authors including terephthalic acid, phthalimide, dimethyl terephthalate, 4-hydroxybenzoic acid, 5-hydroxyisophthalic acid, 3,5-dihydroxybenzoic acid, phenyl isocyanate, phthalic anhydride, 4-aminobenzoic acid, resorcinol, and diphenylamine. They reported that, when used in an injection machine at a weight percentage of 1% based upon the weight of polyethylene terephthalate, 4-aminobenzoic acid, 3,5-dihydroxybenzoic acid and diphenylamine were found to be the most effective additives under laboratory conditions. These authors further postulated that 4-aminobenzoic acid acts as both a free radical scavenger and a hydroxyethyl chain blocker.
U.S. Pat. No. 5,258,233 describes polyester/polyamide blends which have gas barrier properties and through reduction of acetaldehyde concentration in the polyester improve the storage properties of foodstuffs over previously reported blends. The use of low molecular weight partially aromatic polyamides having a number average molecular weight of less than 15,000 or low molecular weight aliphatic polyamides having a number average molecular weight of less than 7,000 is the to be more effective in reducing residual acetaldehyde in polyethylene terephthalate based polyesters than high molecular weight polyamides. However, a recognized problem associated with utilizing a polyamide as an additive in polyethylene terephthalate formulations is that it causes discoloration of the preform due to degradation during the melt extrusion process.
International Patent Publication No. WO 94/29378 teaches a polyester/zeolite admixture which is the to have an excellent gas barrier property and an improved flavor retaining property as well as clarity. Addition of small- or medium-pore zeolites in a critical amount to a polyester is the to reduce the concentration of acetaldehyde in the polyester without producing haze.
In International Patent Publication No. WO 98/18848 there is proposed a process for producing molded articles comprising the steps of:                a) melt reacting at least one glycol and at least one dicarboxylic acid to form a polyester having an I.V. (inherent viscosity) of at least about 0.5 dl/g, wherein the at least one glycol is selected from the group consisting of glycols having up to 10 carbon atoms and mixtures thereof and the dicarboxylic acid is selected from the group consisting of alkyl dicarboxylic acids having 2 to 16 carbon atoms, aryl dicarboxylic acids having 8 to 16 carbon atoms and mixtures thereof in the presence of a catalyst which is substantially free from Co compounds;        b) adding an acetaldehyde reducing additive to the polyester to form a reduced acetaldehyde polyester; and        c) forming the reduced acetaldehyde polyester into shaped articles directly from step b.Such a process can thus be operated as a “melt-to-mold” process. As acetaldehyde reducing additive there can be used any additive known to reduce acetaldehyde. Recommended additives for this purpose include polyamides, polyesteramides, nylon-6 and other aliphatic polyamides, ethylenediaminetetraacetic acid, alkoxylated polyols, bis(4-hydroxyethoxyphenyl)-sulphone, zeolite compounds, 5-hydroxyphthalic acid, poly(ethylene isophthalate), supercritical carbon dioxide, and protonic acid catalysts. Other known scavengers such as polyethyleneimine can also be used.        
Another approach, which is described in U.S. Pat. No. 4,361,681, involves capping of the hydroxyl end groups of polyethylene terephthalate with anhydrides such as succinic anhydride or phthalic anhydride. The use of pyromellitic anhydride for end capping of polyethylene terephthalate has been proposed in U.S. Pat. No. 5,243,020.
European Patent Publication No. 0 878 502 A discloses a stabilizer mixture for thermally stabilizing organic polymers, especially food packages, consisting of (1)—tocopherol, (2) a solid polyhydroxy compound which is selected from the group consisting of triglycerin, ditrimethylolpropane, dipentaerythritol, tripentaerythritol, D-mannitol, D-sorbitol, and xylitol or (3) an acid binding material or a mixture of components (2) and (3). Amongst the polymers which can be thermally stabilized in this way are the to be polyesters, including polyethylene terephthalate. However, no experimental evidence is provided which involves use of polyethylene terephthalate.
U.S. Pat. No. 5,250,333 proposes polyethylene terephthalate modified by incorporation in the polymer structure of an alkoxylated polyol, such as ethoxylated trimethylolpropane. The modified polyethylene terephthalate is described as melt strength enhanced and extrusion blow moldable and is the to be useful for making bottles or containers having hot fill applications. Reduced yellowness is the to be provided.
In U.S. Pat. No. 5,939,516 there is proposed production of a modified polyethylene terephthalate by incorporating in the polymer molecule a polyhydroxyl chain branching agent such as trimethylol propane, mesoerythritol, duclitol (galacticol), adonitol (ribitol), or pentaerythritol.
Addition of a polyamide to reduce the concentration of acetaldehyde in bottles made from polyethylene terephthalate is taught in U.S. Pat. Nos. 5,340,884, 5,650,469, and 4,837,115.
Production of a copolyester by polycondensing terephthalic acid, isophthalic acid and a glycol, followed by a two stage heat treatment to reduce the acetaldehyde content is proposed in U.S. Pat. No. 5,864,005.
Use of primary and secondary antioxidants to reduce the amount of acetaldehyde generated by subjecting polyethylene terephthalate to high temperatures is taught in U.S. Pat. No. 5,874,517. Addition of primary and secondary antioxidants to reduce gel formation in polyethylene terephthalate is suggested in U.S. Pat. No. 5,874,515.
U.S. Pat. No. 5,863,964 proposes addition of dl- -tocopherol to a liner component for a potable fluid container for preventing off-flavors due to the presence of aldehydes in the fluid.
A stabilizing system is proposed in U.S. Pat. No. 5,844,027 for organic material susceptible to thermal, oxidative or/and light induced deterioration. This includes—tocopherol.
There remains, however, a need to provide a polymer additive for incorporation in molding compositions which comprise polyethylene terephthalate, a copolyester thereof, or a blend of one of these with a polyamide, in order to reduce the amount of acetaldehyde formed during processing of such molding compositions.
There is a further need to provide a polymer additive which does not lead to discoloration or haze when polyester molding compositions which consist of or contain polyethylene terephthalate or a copolymer thereof and which contain the polymer additive are subjected to injection molding and/or blow molding.
There is a still further need to provide a process for production from molding compositions containing polyethylene terephthalate of blow molded articles, such as bottles and preforms therefor, which will not release significant quantities of acetaldehyde after formation.